There are 3 possibilities:

1) Russia hacked the election

2) Russia did not hack the election but the intelligence community wrongly believes they did

3) Russia did not hack the election and the intelligence agencies don't believe they did but have decided to lie to the American people for their own reasons.

Some of the reasons to believe they did:

[1] They had the motivation

[2] They have the capability

[3] They have done similar attacks in the past (as has the US)

[4] Russian linked hacking groups like Fancy Bear have been tied to the attacks

[5] The choice of targets and leaked information appeared to benefit Russia.

I think there is enough evidence to point to Russia hacking as the most probable explanation.